Teach me about dual mass flywheels

I have never played with these before. Can they be turned? Mine the front part moves while the back part stays still, is this normal?

No to turning.

There is a reason nearly everyone changes them to single mass.....

It can be done, but it requires specialized tooling to maintain the proper exact gap between the two flywheel surfaces.

https://www.youtube.com/watch?v=gzEaPpGSXU4

There are shops where you can send in yours to be done if you don't mind it being gone for awhile or you can buy a remanufactured one and send yours in as a core.

They're heavy, they suck, remove, replace with single mass. The end.

Aren't there wear concerns with a dual mass flywheel that's been around long enough to need a new clutch?

Manufacturers like porsche (gt3rs) and toyota (2jz, Mine;) loose the dual mass setup in the interest of performance. My 2jz lost a ton of rotational inertia (16lbs iirc) and was compromised zero by the swap from dual to single using 100% toyota factory parts.

iadr wrote: Some do have around 5/8th inch rotation from new.

Is this where it bolts to the crank or at the teeth for the starter?

I would not get rid of a DMF just for the hell of it. They are half of a modern engine's harmonic damping. They are a lot of why modern transmissions shift so nicely, they can make the transmissions lighter duty and still live.

Mind you, part of my B5 Audi idea involves replacing the DMF for an aluminum flywheel, but I respect that it will feel like crap and eat transmissions. Wait, why would I do that again?

They can be stone ground on a mill, just a bit more jigging for the machinist.

I know a friend of mine went from dual to singe on his M3 and it was not a fun car to be in anymore due to the chatter it made.

Replacing a DMF on a MINI with a traditional flywheel will destroy the crank thrust bearings in short order. So not always a good idea.

Some play in them is normal, in fact, the point of a dual mass. If you didn't have any chattering issues it should be good.

My e30 came with a 28lb dual mass flywheel. I bought a used single mass from an earlier e30, and had it shaved down to around 18lbs. As far as I could tell, everything about the single mass was better. Not all cars are the same.

Getting rid of the dual mass flywheel was one of the best things I could have done for my Subaru Legacy GT. Much of what I thought was a laggy e-throttle was the fault of the flywheel. I wonder if some of the cars that have issues with going to a standard flywheel don't have sprung hub clutches. No negatives on my Subaru, and even Subaru stopped using the dual mass flywheel on that car in '07.

Car by car, search a forum specific to the car you are monkeying with. I've done them and it's been glory, or done them and had it turn out annoying. Got rid of the dual mass in my brothers 350z and the rattle / clatter at stop lights and parking lots coming from the transmission was ANNOYING!

So if you get rid of the dual mass flywheel, generally a sprung hub clutch will fix the chatter issue. Non-sprung clutch and a solid flywheel is going to be very noisy. At least on my E30 (M42 318is) I went from the stock dual mass at 28 lbs to a single mass and factory sprung clutch from a 6-cylinder car. I had it machined down to about 15 lbs. No extra rattles, much better response, and only minimal vibration increase. the main reason I needed it was the extra torque capacity due to forced induction, and I'd do it again in a heartbeat. M42s aren't known for great thrust bearings either (180 degree) but I've been running this way for 40-50k miles on a 270k mile engine, so I think every engine is going to take it a bit differently.

My experience was with dads 6.5 liter diesel. The springs and stuff that connected the two halves self destructed. A new flywheel was close to $1000, more than dad felt the truck was worth. We welded the two halves together and had a machinist make the two surfaces true again. It has a few years but only about 10,000 miles on that setup. It still all works but it is noisy. Plus it has a really heavy duty trans so it would take more abuse than a car trans. Just my experience, ymmv.

I work as an engine structural analyst, often doing torsional vibration analysis. If you can visualize a simple spring-mass-damper system from Physics 101, that's basically how crank systems are modeled, but in cylindrical coordinates. In addition to the obvious masses like the outer and inner rings of the crank damper and DMF, the crank and other seemingly "stiff" masses are modeled as really stiff springs with associated damping properties all attached to each other.

This system of dozens of springs, masses, and dampers is being excited by different frequencies at different locations depending on speed and load. Usually there's a resonance or two or more, so the damper and DMF masses and spring rates are "tuned" to cancel out those harsh vibrations and keep the crank from destroying itself or what it's attached to.

Here's a neat visualization:

When you remove or replace part of this system, it's difficult to predict what will happen. This is more of an issue for very highly stressed, high cylinder pressure engines like diesels. In a little gas car engine, the crank system is tuned mainly for NVH. If you change a whole bunch of stuff, like different pistons, conrods, crank, etc, you have an entirely new torsional system and every build will behave a little differently.

My recommendation? If it shakes like E36 M3 after taking it off, put it back on or try a different flywheel ie different mass or stiffness. Also look into crank damper options. Chances are a lighter flywheel will increase natural frequency, possibly pushing it out of the engine operating range. Larger flywheels will resonant a lower frequencies.

YMMV

gearheadE30 wrote: So if you get rid of the dual mass flywheel, generally a sprung hub clutch will fix the chatter issue. Non-sprung clutch and a solid flywheel is going to be very noisy. At least on my E30 (M42 318is) I went from the stock dual mass at 28 lbs to a single mass and factory sprung clutch from a 6-cylinder car. I had it machined down to about 15 lbs. No extra rattles, much better response, and only minimal vibration increase. the main reason I needed it was the extra torque capacity due to forced induction, and I'd do it again in a heartbeat. M42s aren't known for great thrust bearings either (180 degree) but I've been running this way for 40-50k miles on a 270k mile engine, so I think every engine is going to take it a bit differently.

My m42 went from that DMF abomination to a 7lb'er that TMS used to sell. No issues with it since its been in the car for 45k miles, trans doesn't make noise either. No noticeable increase in NVH, but that probably has more to do with the car having solid mounts on about everything for the last 60k miles.

NOHOME wrote: I know a friend of mine went from dual to singe on his M3 and it was not a fun car to be in anymore due to the chatter it made.

I think a lot of this depends on how light a flywheel you pick to replace your dual mass (stock) unit. For instance, I replaced my stock flywheel with a 14.8 pound unit from ACT a few years ago.

https://grassrootsmotorsports.com/project-cars/1997-bmw-m3/fresh-friction/

After the break-in period was over, the flywheel did start to chatter a bit at idle. It's not a big deal though, and it you get annoyed with it---just engage the clutch. I can honestly hardly hear the chatter in my car.

The upside is the car absolutely chomps at the bit now....especially in lower-speed 2nd gear corners. Yes, it's a little more difficult to get moving, but once I became acclimated to the new flywheel--- it became second nature.

I'd do it again in a second. It made my car much more responsive, and fun to drive.

iadr wrote: Since there are a lot of people here who (wrongly) believe there's no harm in staying in the upper 20-30% of the tach,

Wait, wait, are you saying redlining the E36m3 out of an engine is bad for it?!

Come to think of it, the only car i have that's hit the rev limiter ever is my RX8. What about the lower half of the tack, but WOT all the time?

maschinenbau wrote: I work as an engine structural analyst, often doing torsional vibration analysis...

A bit of a tangent, but related. Maybe you can answer a question that I've had for years, about how many Honda kids swap out their OEM harmonic balancer pulleys for solid aluminum ones. There's an enormous number of posts about the consequences of doing so, with half saying that swapping one it will destroy the bearings, while believers say there no problem or downside and that the engine will rev more freely.

What are the facts?

kb58 wrote:

maschinenbau wrote: I work as an engine structural analyst, often doing torsional vibration analysis...

A bit of a tangent, but related. Maybe you can answer a question that I've had for years, about how many Honda kids swap out their OEM harmonic balancer pullies for solid aluminum ones. There's an enormous number of posts about the consequences of doing so, with half saying swapping that in will destroy the bearings, while believers say there no problem or downside and that the engine will rev more freely. What's the truth?

Getting rid of the harmonic damper is definitely no good. Depending on the engine, it may not be enough to make it blow up, but it'll almost certainly accelerate bearing wear, especially at high rpm. And some engines are externally balanced with an unevenly weighted balancer, so removing that would definitely be bad news.

As far as revving more freely, less rotating weight (and under-driving the accessories) will definitely help a bit. But there's certain steps (like deleting the harmonic damper) that just shouldn't be done.

This thread is interesting, and the takeaway I get from anecdotal data is that if an engine was engineered pre-DMF, they work just fine without it, but if the engine and DMF were designed as a system, life sucks with a standard flywheel.

I switched to an aluminum underdrive crank pulley on my track car. I did have my pulley, crankshaft, and lightweight flywheel balanced by a machine shop though.

rslifkin wrote:

kb58 wrote:

maschinenbau wrote: I work as an engine structural analyst, often doing torsional vibration analysis...

A bit of a tangent, but related. Maybe you can answer a question that I've had for years, about how many Honda kids swap out their OEM harmonic balancer pullies for solid aluminum ones. There's an enormous number of posts about the consequences of doing so, with half saying swapping that in will destroy the bearings, while believers say there no problem or downside and that the engine will rev more freely. What's the truth?

Getting rid of the harmonic damper is definitely no good. Depending on the engine, it may not be enough to make it blow up, but it'll almost certainly accelerate bearing wear, especially at high rpm. And some engines are externally balanced with an unevenly weighted balancer, so removing that would definitely be bad news.

Exactly, it really just depends on the engine. I can't broadly say 'yes' or 'no' that removing a damper will destroy your engine. For some engines it's integral to crank stresses. For others it's just NVH. I don't know Hondas enough to answer your forum debate.

If the engine is balanced without needing a damper, it should not really affect bearings. If the damper is part of the balancing, it will definitely impact bearings.